It is generally known that measuring devices which register energy consumption, such as for example electricity meters, water meters or gas meters, are secured by means of metal seals. Also security transport containers, containers and areas to which unauthorized persons have no access are sealed by means of metal seals or other seals, for example self-adhesive seal labels known as “VOID Labels”. A metal seal frequently consists of a security loop made of wire, which then passes through holes of the object to be secured and the point of intersection of which is then enclosed by stamping a piece of lead. A break in the metal seal may only be determined in situ. If the seal was broken open and replaced by a forged one or if the seal broken open beforehand was re-sealed by skillful handling and the application of a suitable tool, this unauthorized access can however no longer be determined unequivocally.
In order to determine whether the casing of an electrical device was opened, so-called “Case-Open-Switches” are also used, the status of which is interrogated and stored within a logic backup battery arrangement.
In order to protect safety-critical electrical devices such as for example terminals, which are used for financial transactions, against an unauthorized access, the use of coded seals is furthermore well-known, which can be affixed to the line of separation between the parts of the casing and monitored electrically.
A coded seal known from U.S. Pat. No. 5,512,738 exhibits a conductivity pattern which is affixed over a line of separation between both parts of the casing and which breaks when the parts of the casing are opened. A monitoring circuit in the device monitors the soundness of this seal. The disadvantage is that the seal can be used only once. A destroyed seal must be removed manually and replaced by a new seal. A further disadvantage is that the seal is affixed from the outside, so that despite coding it is not to be excluded that the monitoring circuit is short-circuited from the outside prior to an improper access.
There are applications in which not only a high safety demand is made, but it is also desired that an opened seal can return to its original state without being destroyed. In this process, it should not be possible to identify the specific status of the seal so easily from the outside. For the maintenance of the functional state of the seal, no auxiliary energy should be necessary so that even in the current-free state of the device, the seal can be verified remotely.
Such an application is present for example in systems which contain or process confidential information and where an unauthorized access is to be detected so as to provide maximum security against forgery. It should also be possible to detect an unauthorized access if the device was in a current-free state for an extended period of time or is still in this state at the time of the check. A further application is evidence when settling warranty claims which is always based on whether it can be conclusively proved that the casing of a device was opened and therefore unauthorized manipulation within the device cannot be excluded.